The research involves the study of natural vocal communication in two species of primates in order to develop parallels between animal communication and aspects of human speech and language. The focal points are the study of syntax in adult vocalizations and the ontogeny of vocal communication and the acquisition of vocal competence in infant monkeys. Studies will be performed using playbacks of calls in normal and altered sequences to cotton-top tamarins to see if they respond differently to calls out of syntax. The intersubstitution of notes involved in alarm calling and in contact calls will be studied in pygmy marmosets to determine if it is the rhythm of calling rather than the structure of the notes that differentiates these two call types. Descriptive data on vocal ontogeny will be gathered from infants to determine how the 8 chirps of adult cotton-top tamarins become differentiated in structure and become applied to appropriate contexts in infants, and the differentiation of 4 types of trill vocalizations will be studied in pygmy marmosets. Several studies will examine the role of social interactions in vocal development. Both male and female tamarins exhibit a suppression of adult hormonal functioning while living in family groups. However, this suppression is released when animals are removed from the family and paired with a mate. Several vocalizations appear for the first time with this change in social environment. A noninvasive way of collecting urinary hormonal samples has been developed and will be used to determine possible hormonal correlates of vocal development. Probes involving stimulating territorial vocal bouts and testing the strength of the pair bond will be used to evaluate how quickly new pairs acquire the adult structure and the adult usage of these calls. The role of imitation and social reinforcement in pygmy marmoset alarm calling will be examined as well as the effects of the changing social environment with the birth of new infants that leads to a vocal regression in older infants. The ontogeny of pygmy marmoset's babbling bouts will be studied to determine if vocal practice is involved. This transition from vocal monopoly to turn-taking in calling will also be studied in these animals. Finally, studies will be made of infant tamarin's ability to discriminate between calls that they cannot yet make, and the ability of pygmy marmosets to identify the voices of socially significant animals will be studied. The results will show the importance of early social interactions on the development of adult communicative competence.